In internal combustion engines a high compression ratio influences efficiency of the internal combustion engine in a positive manner. Compression ratio is typically defined as a ratio of an entire cylinder volume before compression to the remaining cylinder volume after the compression. Internal combustion engines with external ignition, in particular gasoline engines which have a fixed compression ratio only allow to select the compression ratio up to a certain number so that so called knocking under full load of the internal combustion engine is avoided. However, for the partial load range of the internal combustion engine, thus at a lower degree of filling of the cylinders the compression ratio can be selected with higher values without incurring “knocking”. The important partial load range of an internal combustion engine can be improved when the compression ratio is variably adjustable. Systems with variable piston rod length are known for example for adjusting the compression ratio, wherein the systems actuate an eccentrical element adjustment device of a piston rod using hydraulic switch over valves.
A piston rod of this general type is known for example from DE 10 2013 107 127 A1. In the piston rod a hydraulic valve is provided which includes a hydraulic supply connection. The supply connection is provided with a hydraulic pressure. The hydraulic pressure moves a hydraulic piston of the hydraulic valve against a force of a preloaded spring. The hydraulic piston remains unchanged in a stable low pressure position in a low pressure range of the supply connection due to the preload of the spring.
In this stable low pressure position the supply connection is hydraulically connected with a first displacement cavity. Thus, the first displacement cavity empties in bursts towards the supply connection when a displacement cavity pressure due to gas or mass forces at the piston rod or at the eccentrical element is greater than a pressure at the supply connection.
The hydraulic piston includes a piston surface. When a pressure is applied to the piston surface which pressure comes from the supply connection and which is in a high pressure range the hydraulic piston contacts a stop. In this stable high pressure position the supply connection is connected with the second displacement cavity. Thus, the second displacement cavity empties in bursts towards the supply connection when a displacement cavity pressure due to gas or mass forces at the piston rod or at the eccentrical element is greater than a pressure at the supply connection.
Thus, during adjustment the displacement cavity is not filled by the relatively low pressure from the supply connection. Instead the displacement cavities are emptied by the relatively high forces at the piston rod against the relatively low pressure from the supply connection. An eccentrical element lever is actuated by a piston arranged in the displacement cavities, wherein the eccentrical element lever in turn adjusts an effective piston rod length through the eccentrical element and thus controls the variable compression of the internal combustion engine.